How to fill 2 dimensional array using Lambda in Java

Question

I'am trying to create simple 2 dimensional array of double. Using for loop it's not a big deal to make it like:

static public double[][] genMatrix(int n) {
    double mat[][]=new double[n][n];
    for(int i=0;i<n;i++)
        for(int j=0;j<n;j++)
            mat[i][j]=generateDouble();
    return mat;
}

I'd like to do it smoother with a lambda expression and forEach but I'm getting an array full of zeros. Why?

private static double[][] genSquareMatrix(int matrixDimension) {
    double matrix[][] = new double[matrixDimension][matrixDimension];
    Arrays.stream(matrix).forEach(x->Arrays.stream(x).forEach(y-> y = generateDouble()));
    return matrix;
}

By the way, when I use a lambda to print that array there is no problem.

You are getting only zeros because you don't write anything to your arrays. You just iterate all elements, which are default-initialized to zero, and always overwrite the local variable y by generateDouble. — akuzminykh
– akuzminykh, Commented Nov 5, 2020 at 1:22
@akuzminykh I wanted it to be shorter then my instructor version at university but now seems like he know what he's doing — Tom Sevet
– Tom Sevet, Commented Nov 5, 2020 at 1:25
Yes, just keep it simple and use loops in the beginning. The answer that you've accepted is btw. wrong. It addresses a right thing, but doesn't answer your question. — akuzminykh
– akuzminykh, Commented Nov 5, 2020 at 1:26
@akuzminykh Can you explain that further? Java Streams are not modifying their source in any way. Taken from here: Functional in nature. An operation on a stream produces a result, but does not modify its source. For example, filtering a Stream obtained from a collection produces a new Stream without the filtered elements, rather than removing elements from the source collection. — maxemann96
– maxemann96, Commented Nov 5, 2020 at 1:34

maxemann96 · Accepted Answer · 2020-11-05 01:38:54Z

0

Regarding to documentation

Functional in nature. An operation on a stream produces a result, but does not modify its source. For example, filtering a Stream obtained from a collection produces a new Stream without the filtered elements, rather than removing elements from the source collection.

It returns a DoubleStream, which supports

sequential and parallel aggregate operations

This is why it not works.

Edit: The first section was incorrect. Replaced it by a more correct version

edited Nov 5, 2020 at 1:38

answered Nov 5, 2020 at 1:19

maxemann96

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